Network approach for reducing the number of phase shifters in a limited scan phased array

ABSTRACT

An arrangement for scanning of an array by means of controllable phase shifters. A network approach is used to substantially reduce the number of phase shifters required for a limited-scan array. Each phase shifter output is distributed over more than one antenna element of the array. The concept is applicable to both one-dimensional and two-dimensional scanning. The grating lobes of the array factor are allowed to exist in real space but are excluded from the scan region by selecting the subarray spacing. The network is designed to suppress other grating lobes in real space.

United States Patent [191 Nemit Apr.9,1974

[ NETWORK APPROACH FOR REDUCING THE NUMBER OF PHASE SHIFTERS IN ALIMITED SCAN PHASED ARRAY [75] Inventor: Jeffrey T. Nemit, Canoga Park,

Calif.

[73] Assignee: International Telephone and Telegraph Corporation, NewYork, NY.

[22] Filed: Dec. 18, 1972 [2]] Appl. No.: 316,046

[5 7 ABSTRACT An arrangement for scanning of an array by means ofcontrollable phase shifters. A network approach is used to substantiallyreduce the number of phase shifters required for a limited-scan array.Each phase shifter output is distributed over more than one antennaelement of the array. The concept is applicable [52} US. Cl 343/854,343/778, 343/844 [51 1m. (:1. H0lq 3/26, HOlq 21/00 to both Y1ed1mens10nal and two-ilmenslonal scan- 5 Field of Search U 343 77 54 53 77 ning. The grating IObBS Of the array factor are allowed 343/844, 75 4to exist in real space but are excluded from the scan region byselecting the subarray spacing. The network 56] References Cited isdesigned to suppress other grating lobes in real UNITED STATES PATENTSspace 3,725,929 4/1973 Spanos 343/844 7 Claims, 7 Drawing FiguresSUM/22A? O72 rfi l 61/1 2 So Sxfl NETWORK APPROACH FOR REDUCING THENUMBER OF PHASE SHIFIERS IN A LIMITED SCAN PHASED ARRAY BACKGROUND OFTHE INVENTION 1. Field of the Invention The invention relates toscanning antennas as commonly used in the radar arts, and moreparticularly, to the so-called phased array types.

2. Description of the Prior Art Phased arrays find major applicationwhere it is desirable to scan a beam electronically and, therefore, inan inertialess manner, rather than relying on partially or fullyrotatable, mechanical arrangements. The advantages of inertialessscanning are well understood. Such scanning arrangements have beenextensively described in the literature. In the textbook entitled RadarHandbook by Merrill Skolnik, a McGraw-Hill book (1970), Chapter 11 isdevoted entirely to array antennas and the inertialess scanning of thebeams formed by those antennas. FIG. 3 of that chapter described aphased array equivalent to that shown as prior art in FIG. I of thedrawings of the present application. That figure shows the classicalform of phased array operated in accordance with programmed variation ofphase shifters. Each of the N phase shifters controls a correspondingone of the N elements of the array. A power distribution network 7 wouldnormally provide distribution of excitation energy in the transmittingmode, and would also collect the signals through the N phase shiftersand connect them to terminal 8 in the receiving mode.

It will be seen that the number of phase shifters required is equal tothe antenna length divided by the elements spacing. The elementspacings, in turn, are determined by the requirement to avoid gratinglobes in real space, as will be discussed in more detail hereinafter.The maximum element spacing is, therefore, limited to:

Equation 1 The prior art disadvantages include difficulty in managinggrating lobe distribution in practical arrays, as well as the equipmentmultiplicity referred to above.

SUMMARY OF THE INVENTION It may be said to be the general objective ofthe present invention to provide a new approach for substantiallyreducing the number of active elements required for a limited-scanphased array. The concept is applicable to both one-dimensional andtwo-dimensional scanning as will be more fully described.

In accordance with the general concept of the present invention, the Nradiating elements are fed from a subarray interconnecting networkhaving a smaller number M of network ports, as compared to the number Nof antenna elements in the array. Each of the said M subarray networkports is fed from a distribution network, much as in the prior art,through a discrete phase shifter. Accordingly, there are M phaseshifters for the N antenna elements, M being a substantially smallernumber than N. Spacing of subarray inputs is chosen so as to locate thegrating lobe area (in terms of angular coordinates) where the normalizedpattern voltage ratio is minimized.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a prior art form ofphased-array scanning configuration.

FIG. 2 is a'grating lobe and desired lobe location diagram for the FIG.1' arrangement.

FIG. 3 is'a basic block diagram illustrating the subarrayinterconnecting network of a typical implementation of the presentinvention with a reduced number of phase shifters.

FIG. 4 is a grating lobe and desirable lobe location diagram for FIG. 3.

FIG. 5 is a detail showing typical interconnections within the subarrayfeed network of FIG. 3.

FIG. 6 is a normalized pattern diagram of a typical subarray.

FIG. 7 is a typical subarray network feed arrangement'for an area arraywith limited-scan volume in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT input terminal 8 in thetransmitting mode, or providing a collection point for energy combinedin 7 in the receiving mode.

FIG. 2 illustrates the lobe limitations imposed by the well-knownphenomenon of grating lobes in real space. The parameters and symbolsdepicted in FIG. 2 are defined as follows:

6 observation angle 0 scan angle of desired lobe 0,, angle of gratinglobe A wavelength S element spacing U sin 0 U, sin 0,

U sin 0,,

It will be noted that a desired scan region defined by :U is shown. Thenominal or central location of the useful lobe is shown at U and anundesirable grating lobe U, is depicted at a spacing of )\/S from U,,.From this illustration, the interrelationship of element spacing,wavelength and the angular extent of the desired scan region aredepicted.

Referring now to FIG. 3, an idealized illustration of the conceptsinvolved in the present invention is presented in block form. Here the Nantenna elements, typically 9, and 1 1 are comparable to elements 1, 2and 3 of FIG. 1. A subarray interconnecting network is provided,however, and the M phase shifters, typically 12, 13 and 14, are fewer intotal number than the N antenna elements. An input-output terminal 17 iscomparable to terminal 8 of FIG. 1 and the distribution network 16compares with 7 of FIG. 1.

The nature of the subarray interconnecting network 15 will be describedhereinafter. Each subarray input from each of the M phase shifters feedsseveral of the N antenna elements. The spacing between subarray phasecenters is S and is greater than the element spacing. Employingsuperposition with respect to the subarray inputs, the resultant arraypattern is equal to:

PM) UD matical definition of the arrangement of FIG. 3, and Equation IIIis illustrated in the grating lobe limitation diagram of FIG. 4. Thisdiagram of FIG. 4 bears the same relationship to FIG. 3, as did FIG. 2to FIG. 1.

The grating lobes of the array factor are allowed to exist in realspace, but are excluded from the scan region by selecting the subarrayspacing such that:

sin 0,,

Equation IV Accordingly, these extra, or grating lobes, in real spacestill exist as a problem; however, they are inhibited by the idealsubarray pattern illustrated in FIG. 4. The factor reduction in thenumber of phase shifters in this idealized array compared to theconventional limited-scan array is equal to Equation V Hereinafter, apractical technique for approximating the idealized subarray pattern toeffect a significant reduction in the number of phase shifters requiredwill be presented. It should be recognized that if the subarray size isrestricted dimensionally to 5, then it is not possible to synthesize anapproximation to the ideal subarray pattern. A solution to this problemis available, however, through the expedient of having each subarrayinput, with its associated phase shifter, feed overlapping subgroups ofelements by means of interconnecting circuits. Thus, the identificationof the system of the present invention as a network approach" will beseen to be apropos. The increased size of the subarray allows the idealsubarray pattern to be approximated, thereby inhibiting the undesirablegrating lobes.

FIG. 5 simply illustrates the design approach for a linear array with(typical) scan requirement.

FIG. 6 illustrates a subarray pattern (for example, for theconfiguration of FIG. 5) which inhibits the grating lobes by 23decibels. By locating the grating lobes in the identified grating loberange on FIG. 6, by means of selection of the spacing of subarrayinputs, in such a configuration as FIG. 5, the effect is to greatlyreduce the grating lobe amplitude.

It should be recognized that, in the general case, each input (as shownin FIG. 5) could feed a much larger number of elements withinterconnecting circuits to best approximate an ideal subarray pattern.In the FIG. 5 case, each input point (typically 24 or 26) contributespower to each of three antenna elements. Thus, elements l8, l9 and 20are excited from the power divider at 24 and elements 20, 21 and 22 areexcited by power from the power divider 26. In that example, element 20receives energy from both 24 and 26. The components 23, 25 and 27 aretypical couplers for the corresponding antenna elements, and in that waythey act to add power along the branches from the power dividers feedingthe corresponding antenna elements. It will be noted that each of thephase shifters, typically 28 and 29, feeds a corresponding power divider24 and 26, etc., respectively, in this case. Looking typically at thepower divider 26, it will be noted that there is one output S to element21, and two S1 branch outputs to couplers 25 and 27. The relativeamplitude of the S branch is 0.6635 and that of each of the S1 branchesis 0.3348 V2. Each of elements 20 and 22 is, therefore, excited by anamplitude equal to 81/ V2.

The antenna element spacing in FIG. 5 is 0.7M, this and the foregoingpower division parameters combining to produce the effect describedgraphically in FIG. 6, considering the effect of greater subarraycenter-tocenter spacing. l

Thus, it may be said that the general design procedure involvesmaximizing the subarray pattern in the scan region, and minimizing thesubarray pattern in the grating-lobe regions in real space. Thegrating-lobe region is defined as:

grating lobe region U v )tn/ where,

n 1,2, and

v scan region.

The maximum numerical reduction in the number of phase shifters is, ofcourse, limited by the factor G previously defined in Equation V. Therealized reduction will depend on the degree of suppression of thegrating lobes required, and the maximum complexity allowable for theinterconnecting network. It should also be emphasized that the scanregion can be shifted from broadside. In that case, the reduction factoris even larger. In that instance,

G l sin 0,, /sin 0,, sin 0,,

where,

v sin 0,, sin 0,, scan region.

Such a shift is accomplished by appropriate phase shifter programming.

To illustrate the utility of the novel approach of the presentinvention, consider the case of a scan requirement taken to be In thatcase, the maximum reduction in the number of phase shifters is 3.35 overthat required in the conventional linear array of FIG. 1. The relativelysimple interconnecting network illustrated in FIG. 5 results in atwo-to-one reduction in the number of phase shifters, as compared to theaforementioned conventional phased array. It may be said that FIG. 5 isa linear array of overlapping subarrays, elements 20, 21 and 22comprising the antenna elements of a typical subarray therein. Lookingat each subarray as an entity, it will be realized that the effect ofgreater element spacing is achieved from subarray center to subarraycenter, insofar as grating lobe generation is concerned.

The network approach herein described for reducing the number of phaseshifters in a limited-scan array can readily be extended to an areaarray which scans in both planes (horizontal and vertical, for example).The subarray in this case comprises an area rather than a single lineardimension. Accordingly, the general design procedure is to maximize thesubarray pattern in the scan volume, and minimize the subarray patternin the grating lobe region of real space. A subarray feed network for anarea array with a triangular arrangement of elements is illustrated inFIG. 7. This configuration is well suited for scanning a maximum of 10off broadside in any scan plane, while providing 23 decibels ofsuppression of the undesirable grating lobes.

Referring now to FIG. 7 specifically, seven main elements 30, 31, 32,33, 34, 35 and 36 are illustrated. These main elements are comparable tothe elements such as 19 and 21 in FIG. 5. TI-Ie coupled elements i1-lustrated include 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47 and 48.That is, each of these elements is comparable to elements such as and 22in FIG. 5, in that they are excited by a pair of S1 branches taken fromcouplers at the element locations and located similarly in respect tothe coupled and main elements, as depicted in FIG. 5. In FIG. 7, S, 1.00voltage ratio and S1 0.576 X \/2 voltage ratio. Signals are added atcoupled elements through the use of a hybrid at each coupled elementlocation, these hybrids being similar to the couplers 23, and 27,illustrated on FIG. 5. A significant difference, as compared to FIG. 5,is that in the case of each main element, for example, element 30, thereare six branches from the power divider feeding the S, signal to element30. These branch signals are represented by the S1 signals provided tothe coupled elements at 37, 38, 39, 40, 41 and 42 via the hybridcouplers also coupling in signal from the corresponding power dividersfeeding elements 32, 33, 34, 35, 36 and 31, respectively. Thus, theconfiguration of FIG. 7 is an area form of that depicted at FIG. 5.

To summarize, this disclosure will be seen to have presented a newtechnique and structure for limitedscan phased array construction,substantially reducing the number of active devices required in either alinear or area type array.

What is claimed is:

l. A scanning phased array antenna system having a predetermined numberN of antenna elements, and a distribution network having a common inputterminal and a predetermined number of distribution ports M, where M isa smaller number than N, comprising:

M phase shifters each connected at its input discretely from acorresponding one of said M distribution ports;

a subarray interconnecting network having N output ports and M inputports, each of said N output ports being connected discretely to acorresponding one of said antenna elements, and each of said M inputports being connected discretely to the output of a corresponding one ofsaid phase shifters;

divider means within said subarray interconnecting network for dividingthe power input from each of said phase shifters into a primary branchand a plurality of secondary branches, said primary branch beingconnected to a selected one of said antenna elements and said secondarybranches feeding an equal number of antenna elements adjacent on eachside of said selected element;

and means associated with each antenna element fed from one of saidsecondary branches for combining feeds from said branches providing feedthereto from others of said phase shifters.

2. A scanning phased array antenna system comprising:

anarray of antenna elements comprising a first group of main elementsand a second group of spaced.

coupled intermingled elements comprising a subarray;

means including a plurality of discretely controllable phase shiftersfor providing excitation to corresponding ones of said main elements;

a plurality of power dividers connected, one to the output of each ofsaid phase shifters, said dividers each having a main output and aplurality of branch outputs each providing power output at apredetermined fraction of the power supplied by the corresponding phaseshifter, each of said main outputs being connected to an antenna elementof said first group of main elements;

and a plurality of coupler means one of which is discretely connected toeach element of said subarray, said couplers also being connected to mixpower from at least one of said branch outputs from at least each ofsaid power dividers corresponding to the adjacent main elements on eachside of each of said subarray elements.

3. Apparatus according to claim 2 in which said array is a linear array.

4. Apparatus according to claim 2 in which said array comprises a lineararray of plural subarrays, said main elements are first alternateelements of said array, and said subarrays comprise at least one elementon each side of each of said main elements.

5. Apparatus according to claim 4 in which said subarray comprises oneelement on each side of said main element, and said main element.

to a pair of adjacent, but oppositely disposed, main elements.

7. Apparatus according to claim 4 in which said divider is arranged toapportion the power in said branch outputs with respect to said mainelements to produce a predetermined subarray pattern, the phase centersbetween said subarrays being greater than the element spacing of saidarray.

1. A scanning phased array antenna system having a predetermined numberN of antenna elements, and a distribution network having a common inputterminal and a predetermined number of distribution ports M, where M isa smaller number than N, comprising: M phase shifters each connected atits input discretely from a corresponding one of said M distributionports; a subarray interconnecting network having N output ports and Minput ports, each of said N output ports being connected discretely to acorresponding one of said antenna elements, and each of said M inputports being connected discretely to the output of a corresponding one ofsaid phase shifters; divider means within said subarray interconnectingnetwork for dividing the power input from each of said phase shiftersinto a primary branch and a plurality of secondary branches, saidprimary branch being connected to a selected one of said antennaelements and said secondary branches feeding an equal number of antennaelements adjacent on each side of said selected element; and meansassociated with each antenna element fed from one of said secondarybranches for combining feeds from said branches providing feed theretofrom others of said phase shifters.
 2. A scanning phased array antennasystem comprising: an array of antenna elements comprising a first groupof main elements and a second group of spaced coupled intermingledelements comprising a subarray; means including a plurality ofdiscretely controllable phase shifters for providing excitation tocorresponding ones of said main elements; a plurality of power dividersconnected, one to the output of each of said phase shifters, saiddividers each having a main output and a plurality of branch outputseach providing power output at a predetermined fraction of the powersupplied by the corresponding phase shifter, each of said main outputsbeing connected to an antenna element of said first group of mainelements; and a plurality of coupler means one of which is discretelyconnected to each element of said subarray, said couplers also beingconnected to mix power from at least one of said branch outputs from atleast each of said power dividers corresponding to the adjacent mainelements on each side of each of said subarray elements.
 3. Apparatusaccording to claim 2 in which said array is a linear array.
 4. Apparatusaccording to claim 2 in which said array comprises a linear array ofplural subarrays, said main elements are first alternate elements ofsaid array, and said subarrays comprise at least one element on eachside of each of said main elements.
 5. Apparatus according to claim 4 inwhich said subarray comprises one element on each side of said mainelement, and said main element.
 6. Apparatus according to claim 2 inwhich said array is an area type array in which said main elements aredistributed over a surface in a regular pattern, said subarray comprisesan element spaced on a line between each of said maiN elements, saidpower divider branch outputs are six in number corresponding to sixsubarray elements comprising the nearest antenna elements surroundingeach of said main elements, and each of said coupler means beingconnected to a branch output from the power dividers corresponding to apair of adjacent, but oppositely disposed, main elements.
 7. Apparatusaccording to claim 4 in which said divider is arranged to apportion thepower in said branch outputs with respect to said main elements toproduce a predetermined subarray pattern, the phase centers between saidsubarrays being greater than the element spacing of said array.